Optical trapping may be used to catch and hold small particles (micron size and smaller) for study, and has proven extremely useful in the study of biological samples, including viruses, bacteria, living cells and organelles. Once trapped, the object may be imaged using a CCD array. However, a CCD array is unable to respond to the relatively rapid and random Brownian motion of the object within the trap. Accordingly, quadrant photodiodes with MHz readout rates are generally employed when monitoring the position of an object held within an optical trap.
There are several known methods for monitoring the position of an object within an optical trap. A popular method is to collect forward- or back-scattered light, from either the trapping beam or a secondary light source, with a condenser lens, the back focal plane of which is projected onto a quadrant photodiode. Lateral displacement of the optically trapped object is monitored by comparing the signals of pairs of quadrant diodes (e.g. a comparison of the top two and bottom two diodes provides information regarding vertical motion), whilst axial displacement is monitored through changes in the sum of the diode signals.
A fundamental problem with quadrant photodiodes is that they are suitable only for measuring the lateral and axial displacement of geometrically simple objects, and in particular spherical objects. For objects having irregular shapes, it is generally impossible using quadrant photodiodes to decouple lateral and axial displacements. To this end, many samples are first attached to a spherical bead, which is then held in the optical trap.
Additionally, quadrant photodiodes are impractical when monitoring multiple trapped objects using standard incoherent illumination since this requires undesirable splitting of the imaging light as well as the means to accurately move the relative positions of the quadrant arrays. In particular, when an object is held within a moving trap, the movement of the quadrant array must be precisely correlated with that of the moving trap, otherwise the measured position of the object within the moving trap will be inaccurate.
A sizeable CCD array can, of course, image irregular shaped objects and track the movement of an object without necessarily moving the array. However, owing to the relatively slow readout speeds, the CCD array is unable to discern Brownian motion. Consequently, the resulting thermal noise makes the CCD array unsuitable for many applications, including sensitive force measurements.